Uterine fluid from normal and delayed implanting mice has been shown to inhibit RNA synthesis by blastocysts in vitro; the active principal appears to be dialyzable and heat stable. Attempts will be made to purify the active factor by column chromatography. Purified inhibitory factor will be treated with agarose bound protease, RNase and Beta-galactosidase to determine whether or not it can be inactivated by these enzymes. In addition, uterine fluid will be analyzed for trypsin and chymotrypsin activity and, if possible, for inhibitors of these enzymes. If the enzymes are found in the uteri of normal or delayed implanting mice, their effect on embryonic metabolism will be determined in vitro. Studies on synthesis of uterine proteins will be conducted with double isotope methods. C14 and H3 labelled uterine fluids will be electrophoresed and C14/H3 ratios will be calculated to determine whether or not specific proteins are synthesized or lost in delayed implantation.